The effect of an IV Melatonin infusion of the pituitary LH response to LHRH was studied in five young men. Melatonin (30 micrograms min; total dose, 7.2 mg) was infused for a 4-h period, 2h before and after a LRH stimulation (single IV 150 microgram dose). Each subject's control response to LRH was obtained previously. During the Melatonin infusion, supraphysiological concentrations of Melatonin (20 times) were documented using a specific RIA. All five subjects had a LH rise after LRH stimulation, and this response was not affected by the Melatonin infusion. These results indicate that an acute constant infusion of a pharmacological amount of Melatonin does not suppress LRH-induced LH release from the pituitary in men. In addition, no change in sleepiness and behavior was found.